1. Field of the Invention
This invention relates to differential line driver circuits and, more particularly, to a CMOS differential line driver circuit with improved short circuit protection.
2. Prior Art
Line driver circuits are used to drive transmission lines. If the output terminal of such a device is shorted to ground or is coupled through an excessively low impedance to ground, excessive current is drawn from the output terminal of the line driver. In order to prevent the output line driver from being damaged, it is necessary to detect a short-circuit fault condition and take action to prevent the line driver from being damaged. Various schemes are available in the prior art for such short-circuit detection and protection. These schemes may be classified into one of two categories. The first category is for short-circuit protection schemes which sense an excessive amount of output current such as occurs when a short-circuit fault is present. The other technique involves various voltage-sensing techniques to detect a short-circuit condition.
Current sensing techniques in the prior art utilize a resistor connected in series between the primary power control device and the load. The resistor value is generally low and a relatively small voltage is developed across that resistor by the load current to provide a voltage proportional to the load current. When the load current exceeds a predetermined value, protection and/or disabling circuits are energized.
An example of a current detection scheme is shown in U.S. Pat. No. 3,944,889 which distinguishes between a true short circuit load condition and a condition in which short duration surges of high current are supplied to a load, which surges are caused, for example, by switch contact bounce or which results from the nature of the load, for example transient characteristics when the load is a tungsten filament lamp. When the current exceeds a predetermined level, the voltage detector provides an output signal which gates a clock signal into a binary counter. If the short-circuit condition persists, a predetermined number of clock pulses are counted to provide a binary word which is detected to trigger a latch. The latch is set to disable a switch supplying current to the load. The binary counter, in effect, serves as a delay function to verify that the short-circuit condition is a true short-circuit condition, rather than a transient condition.
Another example of a current detection scheme is shown in U.S. Pat. No. 4,322,690 which discloses a short-circuit protection scheme for an audio amplifier which utilizes a current-measuring resistor in series with each of a pair of push-pull output transistors. A feedback transistor is connected in parallel with one of the measuring resistors and is connected to the control electrode of a thyristor. The thyristor is triggered to disable a preamplifier when excessive current is drawn by the output transistor.
An example of a voltage-sensing protection circuit is disclosed in U.S. Pat. No. 4,291,357 in which an output voltage is sensed and the output device is shut off when the output voltage falls below a predetermined trip voltage. This protection circuit is disabled during circuit start-up or during rise time of a signal. This patent points out a problem which may exist with short circuit protection systems. This problem is the so-called false trip problem which may occur during start-up, that is when power is first applied to a circuit or during rise time, that is when the output voltage is required to switch from a low level to a high level. U.S. Pat. No. 4,291,357 avoids the false trip problem by providing a greater delay for an input signal to a transistor which disables the amplifier in comparison with the input delay for a signal in the signal path to the output amplifier. The difference in delay time is utilized to disable the protection circuitry until the output voltage has risen to a value above a predetermined trip voltage level. The signal delay for the scheme was obtained by using the recovery time needed for a transistor to recover from saturation. This technique depends on a parameter, namely, recovery of a transistor from saturation, which in practice may be difficult to control precisely.
Line driver circuits typically include a so-called pull-up transistor, which provides high output signal levels, and a so-called pull-down transistor, which provides low output signal levels. The current delivered by a pull-up transistor depends on the load impedance. For a very low impedance or for a short-circuit fault condition, a pull-up transistor begins to supply current as the output voltage goes from a low state toward a high state. If excessive current is drawn through a pull-up transistor, it may be damaged. Note that a pull-down transistor serves as a current sink for the output load and does not need short-circuit protection.
Differential line driver signals are frequently used to drive transmission lines and one is called a true signal and its complement is called a false signal. Differential line driver circuits provide two differential output signals, that is, signals which are equal in magnitude but of opposite phase. These signals are complementary. Differential line-driver circuits typically use two separate line driver circuits, one fed with a true input signal and the other fed with a false input signal. These circuits are required to have substantially the same propagation delay. Differences in propagation between the two signals is measured by a skew characteristic which measures the time difference for a signal transition at the output of a differential line driver. The time difference is measured between the 50% amplitude points of each of the signals. A good skew characteristic is less than one nanosecond.
For line driver circuits, conventional current-sensing short-circuit protection schemes typically have a current-sensing resistor in series with the pull-up transistor because a short-circuit condition causes excessive current to be drawn through the pull-up transistor, while the pull down transistor does not require a current-sensing resistor. Insertion of a resistance in series will increase the pull-up rise time, while the pull-down fall time is unaffected. A differential line driver system uses two line driver circuits, one providing a true output signal and the other providing a complementary false output signal. For a given signal transition, such as for example a low to high transition, the pull-up transistor of the one circuit is active and the pull-down circuit of the other circuit is active. The one circuit has a slower time constant than the other. Consequently, the skew characteristic is not optimum because the one circuit, with a series current-sensing resistor, reaches its 50% amplitude value slower than the other circuit, without a series resistor. Therefore, it should be appreciated that conventional current-sensing schemes for short-circuit protection adversely affect the operating characteristics of line driver circuits In particular, differential line driver systems, using line driver circuits operating in complementary modes, are adversely affected by use of series current-sensing resistors.